Wireless devices or terminals for communication are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server, such as server providing video streaming service, via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, computers, or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
A cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNodeB (eNB), NodeB, B node, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations. The base stations and wireless devices involved in communication may also be referred to as transmitter-receiver pairs, where the respective transmitter and receiver in a pair may refer to a base station or a wireless device, depending on the direction of the communication. Two wireless devices involved in D2D communication may also be referred to as a transmitter-receiver pair. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to a wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for communication with terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
In LTE two mechanisms may be used to detect and correct the errors. The first mechanism Hybrid Automatic Repeat reQuest (HARQ) is implemented to correct the error packets in the PHY layer. Furthermore, there may be some packets that are still left with errors but may be acceptable to some applications. Hence, these are passed to upper layers. The second mechanism Automatic Repeat reQuest (ARQ) is implemented in the higher RLC layer which handles these residual errors. It either repairs those errors or discards the packets.
ARQ works at RLC layer. If the received data has an error detected by ARQ then it is discarded, and a new re-transmission is requested from the sender.
HARQ works at PHY layer but is controlled by Medium Access Control (MAC) layer. If the received data has an error then the Receiver buffers the data and requests a re-transmission from the sender. When the receiver receives the re-transmitted data, it then combines it with buffered data prior to channel decoding and error detection.
RLC ARQ
3GPP specifies E_UTRAN RLC protocol. An RLC entity such as receiving side and transmitting side may be configured to perform data transfer in one of the following three modes: RLC Transparent Mode (TM), RLC Unacknowledged Mode (UM) or RLC Acknowledge Mode (AM). The ARQ part is only supported in the RLC Acknowledge Mode. The ARQ in RLC is an error-control method for data transmission that uses acknowledgements, such as messages sent by the receiver indicating that it has correctly received the packet, and timeouts such as specified periods of time allowed to elapse before an acknowledgment is to be received, to achieve reliable data transmission over an unreliable link. If the sender does not receive an acknowledgment before the timeout, it re-transmits the packet at given intervals until the sender either receives an acknowledgment or if it exceeds the maximum number of retransmissions. If the maximum number of re-transmissions is reached, RLC will indicate that to upper layer, such as Radio Resource Control (RRC).
The mode used by an RLC entity is decided by Radio Resource Control (RRC) protocol for each radio bearer and it is signaled from eNB to UE at radio bearer setup. The RRC protocol layer is e.g. comprised in UE and eNB, and it is part of LTE air interface control plane.
An RLC UM transmitter may segment and concatenate Service Data Unit (SDU)s into RLC PDUs to be sent to lower layer, as MAC. The RLC UM receiver reassembles the SDUs. This gives a higher flexibility to the lower layers which can adapt to whatever transfer possibilities are provided by a scheduler and the radio link. RLC SDUs which have missing segments due to errors in lower layers are discarded in RLC UM receiver, which also discards duplicates.
The RLC AM mode supports ARQ in addition to the functions described above. The ARQ part makes it possible to have missing PDUs or portions of PDUs to be retransmitted. The transmitting side of a RLC AM entity receives status PDUs from its receiver peer informing about which PDU sequence numbers have been received and which have not been received. A PDU that has not been reported as received will be retransmitted after a configured amount of time. Failure to deliver SDUs to higher layers in AM results in RLC delivery failure which is determined by a threshold defining maximum number of retransmissions in the transmitter side which in turn results in Radio Link Failure (RLF).
As mentioned above, adding RLC ARQ on top of HARQ decreases the probability of packet loss a lot. Here on top of means on a layer above. This is since RLC Status PDUs are Cyclic Redundancy Check (CRC)-protected and it is therefore very unlikely that an AM RLC entity misinterprets the feedback from its peer, something that may happen more frequently with the single bit feedback used by HARQ. RLC Status PDU here means feedback from its RLC peer.
However, the cost is an increased delay and more schedulings are required, such as e.g. for retransmissions and feedback. The drawbacks are that retransmissions on RLC level takes a relatively long time compared to HARQ retransmissions since the RLC retransmission timer needs to be long enough to give MAC a chance to deliver the RLC PDUs to avoid unnecessary RLC retransmissions and further that resources used in the initial transmit attempt are wasted as opposed to HARQ retransmission where soft combining is used. Soft combining means combining data received from previous transmissions. With this in mind, usage of AM RLC is typically restricted to services that are more tolerable to delay than to packet losses.
In LTE the Signalling Radio Bearers (SRBs) are configured to use RLC AM.
Discontinuous Reception (DRX)
The “always-on” type of behavior that is arriving with smartphone UEs adds much strain on the battery economy in the UE. There are different methods in LTE to limit the power consumption in the UE; one such method being DRX.
A UE is in RRC_CONNECTED when an RRC connection has been established between the UE and an eNB. If no RRC connection is established, the UE is in RRC_IDLE state.
DRX may be applied both in RRC_IDLE and in RRC_CONNECTED. The principles are the same. The description hereafter applies to DRX during RRC_CONNECTED since that maps best to the above mentioned “always-on” type of behavior.
DRX is a method to reduce battery consumption in the UE by allowing the UE to stop monitoring the Physical Downlink Control Channel (PDCCH), i.e., it can turn off the receiver during lengthy times and just discontinuously listen during short on-duration phases. The time periods where the receiver is turned off are configured by the network and acknowledged by the UE. The reoccurring periods of the on-duration phase is illustrated in FIG. 1.
DRX Active Time. The standard, 3GPP TS36.321, defines the Active Time as the aggregated phases while either:                onDurationTimer or drx-InactivityTimer or drx-RetransmissionTimer which is used to supervise any DL retransmission that UE expects, or mac-Contention ResolutionTimer which is used to supervise the completion of random access, is running; or        a Scheduling Request for UL transmission is sent on Physical Uplink Control Channel (PUCCH) and is pending; or        an uplink grant for a pending HARQ retransmission can occur and there is data in the corresponding HARQ buffer; or        a PDCCH indicating a new transmission addressed to the Cell Radio Network Temporary Identifier (C-RNTI) of the UE has not been received after successful reception of a Random Access Response for the preamble not selected by the UE.        
Voice Over LTE (VoLTE)
In LTE all packets are delivered using the Internet Protocol (IP). This means that also traditionally circuit switched services such as conversational voice will make use of fast scheduling and will be provided using Voice over IP (VoIP). In a typical VoIP arrangement a voice encoder on the transmitter side encodes the speech into packets with the typical speech duration of 20 ms, mandated by GSM Association (GSMA) IR.92.
GSMA IR.92 is a Permanent Reference Document (PRD), i.e. a reference instead of a standard, defining the minimum recommended set of features and functions in IMS, EPC and LTE RAN to deliver a VoIP based voice service. GSMA PRD IR.92 is adopted by the industry to ensure interoperability between network nodes as well as securing the service delivery.
VoLTE enables LTE networks to provide voice services. The mouth-to-ear delay introduced by the transport scheduling and transmission of the VoLTE packets is one of the main factors determining the experienced speech quality. This causes a relatively tight delay budget for VoLTE to ensure good speech quality. The normal delay budget in the eNB is 80 ms. Up to the limit of the delay budget the speech quality is good. This means that it in general is sufficient to schedule a VoLTE service once every 40 ms and bundle two packets. Such a scheduling method allows for a good balance between efficient usages of resources, good speech quality and enhanced battery performance.
When DRX is combined with VoLTE the UE is configured to monitor the PDCCH during one or more subframes with a 40 ms period. Since speech packets are produced once every 20 ms this DRX configuration allows for transmission of two speech packets every 40 ms. FIG. 1 illustrates VoLTE scheduling in uplink when configured to monitor the PDCCH during one or more subframes with a 40 milliseconds (ms) period.
Please note that both onDurationTimer and inactivity Timer defines periods where the UE is Awake. It is the time when no timers, between the onDurations, are running that the UE is asleep.
The advantage of this configuration is that two packets are scheduled per scheduling opportunity, allowing efficient use of PDCCH and other scheduling resources, while the scheduling delay is still less than the delay budget. However, the tight delay budget of VoLTE does require that the wake period is utilized for scheduling of the VoIP packets that are available. If the opportunity is missed, an additional delay of 40 ms is reducing the possibility to transmit the packets within the packet delay budget.
Handover
In wireless telecommunications, the term handover refers to the process of transferring an ongoing call or data session in a UE from one channel connected to the core network to another channel.
In telecommunications there may be different reasons why a handover might be conducted such as e.g.:                When the UE is moving away from the area covered by one cell and entering the area covered by another cell the call is transferred to the second cell in order to avoid call termination when the UE gets outside the range of the first cell.        When the capacity for connecting new calls of a given cell is used up and an existing or new call from a UE, which is located in an area overlapped by another cell, is transferred to that cell in order to free-up some capacity in the first cell for other UEs, who can only be connected to that cell.        In non-CDMA networks when the channel used by a UE becomes interfered by another UE using the same channel in a different cell, the call is transferred to a different channel in the same cell or to a different channel in another cell in order to avoid the interference.        Again in non-CDMA networks when the UE behavior changes, e.g. when a fast-travelling UE, connected to a large, umbrella-type of cell, stops then the call may be transferred to a smaller macro cell or even to a micro cell in order to free capacity on the umbrella cell for other fast-traveling UEs and to reduce the potential interference to other cells or users, this works in reverse too, when a user is detected to be moving faster than a certain threshold, the call can be transferred to a larger umbrella-type of cell in order to minimize the frequency of the handovers due to this movement.        In CDMA networks a handover, see further down, may be induced in order to reduce the interference to a smaller neighboring cell due to the “near-far” effect even when a UE still has an excellent connection to its current cell.        
The most basic form of handover is when a UE call in progress is redirected from its current cell, referred to as source cell, to a new cell, referred to as target cell. In terrestrial networks the source and the target cells may be served from two different cell sites or from one and the same cell site. In the latter case the two cells are usually referred to as two sectors on that cell site. Such a handover, in which the source cell and the target cell are different cells, even if they are on the same cell site, is called inter-cell handover. The purpose of inter-cell handover is to maintain the call as the UE is moving out of the area covered by the source cell and entering the area of the target cell.
In LTE the UE normally triggers a handover by sending a measurement report, via the RRC protocol, including a specific Event, like EventA3. The different events are defined by 3GPP, TS 36.331. The eNB will then act on this measurement report, depending on situation and configuration, and prepare the target cell for an incoming handover. The source cell sends out a Handover Command, which may be referred to as RRC Connection Reconfiguration to the UE, via the RRC protocol, comprising all the information the UE needs to perform a successful handover to the target cell.
The transmission of both the Measurement Report and Handover Command such as the RRC Connection Reconfiguration are transmitted using a Signaling Radio Bearer (SRB) which is configured to use RLC ARQ, in AM.
Battery Performance
For a good end user experience it's important with a good battery performance, i.e. the battery should last until the next charging opportunity. Packet intensive services with a continuous flow of packets will put higher demands on the battery performance.
A way to increase the battery performance is to use DRX, where the UE is allowed to turn off the receiver and hence save battery. To be able to use this on services as voice and conversational video where a lot of packets are transferred with small inter arrival time, short onDuration and inactivity times in DRX is needed. These short times will on the other hand increase the risk of the UE being in a DRX sleep state when it is time to send or resend the Handover Command and the eNB will have to delay the transmission until the next onDuration i.e. a drx cycle later.
A problem is related to that RLC retransmissions may miss onDuration opportunities to transmit. Missed on Durations imply an increased delay of the data transmission from the eNB when using DRX, which may result in a negative impact on the ongoing session or in worse case a call drop due to a delayed procedure. If the delay hits RRC signaling data transmission for the handover procedure the impact of extra delay is much more severe.